Conventional switching devices typically comprise an input lever and an output lever coupled by a stored-energy spring. The input lever and the output lever, sharing a common axle, rotate freely of one another.
The output lever has a cammed surface at its base which is generally rounded and concave relative to the common axle. The cammed surface also has a left edge and a right edge which are generally convex relative to the common axle.
A latching device engages the cammed surface of the output lever, and specifically, it selectively engages one of the left edge and the right edge of the cammed surface, thereby securing the lever in place and preventing it from rotating freely. The latching apparatus includes a left latch and a right latch. The left latch rotates on a left pivot and is connected to a latch spring. The right latch rotates on a right pivot and is also connected to the latch spring. When positioned on the quick-break device, the latching device forms a V-shape. The latching spring urges the left latch and the right latch toward one another so that the left latch and the right latch tend to pinch the cammed surface.
The left latch engages and secures the left edge of the cammed surface when the output lever is fully rotated in the counterclockwise direction. The right latch engages and secures the right edge of the cammed surface when the output lever is fully rotated in the clockwise direction.
The latching device is engaged by a base section of the input lever. For example, as the input lever rotates in a counterclockwise direction, the base section comes in contact with either the right or left latch. The force exerted by the base tends to force the right latch away from its seat in the edge of the cammed surface, thereby releasing the output lever.
When, for example, a changeover from an odd-numbered tap to an even-numbered tap is desired, the input lever is mechanically rotated in a counterclockwise direction to an interim position. As the input lever rotates in a counterclockwise direction, the stored-energy spring is stretched, thereby transferring potential energy to the stored energy spring.
The output lever is held in place by the right latch, which, due to the force exerted by the latch spring, engages the right edge of the cammed surface. As the input lever rotates, a section of its base forces the right latch away from the right edge of the cammed surface. When the force exerted by the base of the input lever on the right latch exceeds the force exerted by the latch spring, the right latch passes over the right edge of the cammed surface, and the output lever is free to rotate in a counterclockwise direction, i.e., the output lever is tripped.
At this point, the potential energy stored in the stored-energy spring is released, and the output lever is pulled in a counterclockwise direction toward the input lever. The output lever continues to rotate until the left edge of the cammed surface is captured by the left latch. At this point in time, the output lever is now on the other side of the switch, in the even-numbered tap position, and the changeover process is complete.
There are a number of problems with conventional switching devices. First, the output lever is typically tripped before the stored energy spring has been stretched to its maximum limit; hence, it is extremely difficult with conventional devices to delay the switching operation until the spring has been fully stretched and a sufficient amount of potential energy is stored in the spring. Second, the operator is unable to precisely control the exact angular position of the input lever before the output lever is tripped. Third, due to the typically large cammed surface that is on the base of the output lever, the size of these conventional switching devices is typically larger than desired.
A number of attempts have been made to solve the problems discussed above. For example, Japanese Provisional Patent Publication TOKUKAISHO No. 108219 of 1981 attempts to solve the problems by using an output lever having enlarged cams. According to this patent, the enlarged cams 107 and 108 increase the space between the latching levers 109 and 110. The enlarged cams enables the output lever to receive sufficient starting torque when the output lever has started. The initial breaking speed of the changing contacts is boosted thereby preventing the output lever from being delayed mid-course. Problems exist with this disclosure, however, because the enlarged levers require additional space which is not readily available and the production costs are high.
Another solution was presented in Japanese Provisional Patent Publication TOKUKAISHO No. 132021 of 1980. This reference discloses a coiled compression spring which is utilized as a stored-energy spring, wherein the spring is disposed axisymmetrically between an input case and an output case of a flat, U-shaped device which retains 30 the spring inside the U-shape. A driving source (motor) provides the driving power to compress the spring by moving the input and output cases in the direction of the spring axis. A latch-tripping arm which is fixed to the input case is simultaneously moved. A cam formed on the rim of a circular plate receives the rotational force from the spring. The arm of a latching lever that supports a cam follower with which it engages through the output case is integrated into the drive shaft of the changeover contacts. As a result the quick-break action begins at the position of maximum deformation of the spring.
However, there are a number of problems associated with the disclosed technique. Generally, the structural dimensions around the latching levers are too large. This is due, in part, to the fact that the cams are enlarged.